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Coal-Based Carbon Foams. Influence of the Precursor Coal
Montserrat Calvo, Roberto Garca, Sabino R. Moinelo Instituto
Nacional del Carbon (CSIC), Apartado 73, 33080 Oviedo, Spain
Email: [email protected] Abstract Carbon foams were obtained
from several bituminous coals with different plasticity and
volatile matter content by a two-stage thermal process. The first
stage, a controlled carbonisation treatment under pressure at
450-500 C, is responsible for the final textural properties of the
foam. In the second stage the carbonisation product was baked at
1100 C. The foams produced display a macroporous texture with
plasticity, volatile matter content and maceral composition of the
precursor coals having an influence on the apparent density and the
pore size of the resultant porous products.
Keywords Carbon materials, Pore structure, Macerals INTRODUCTION
Carbon foams are lightweight (0.2-0.8 g cm-3) and exceptionally
strong cellular materials that have thermal and electric
conductivity adjustable by the foaming thermal treatment. Their low
production costs and flexible physical properties make them ideal
for a wide range of applications, including thermal management,
electromagnetic interference, acoustic shielding and batteries and
fuel cell components (Gallego and Klett, 2003; Jang et al., 2006;
Min et al., 2007).
Carbon foams were first developed as reticulated glassy carbons
(Ford, 1964) from thermosetting organic polymer foams through a
thermal treatment. In the 1990s, research focused primarily on
producing carbon foams from alternative precursors such as pitches
and coals. Graphitized carbon foams are most appropriate to produce
high strength and thermal and electrical conductivity because of
the interconnected graphitic ligament network (Gallego and Klett,
2003; Jang et al., 2006; Klett et al., 2000; Min et al., 2007).
However, carbon foams made from coal have hardly been studied
(Calvo et al., 2005; Chen et al., 2006; Rogers et al., 2001), in
spite of being an attractive more economical alternative to
traditional materials since their structural properties made them
perfectly useful in numerous applications, when very high
conductivity is not required.
In this work, carbon foams were obtained from ten bituminous
coals through a simple thermal procedure. Unlike previous works,
not only high volatile bituminous coals were used as feedstock, but
also medium and low volatile ones. Neither previous modification of
coals nor further stabilization step of the green foams was carried
out. This leads to a faster and cheaper production process as
compared to foams prepared from other feedstock. The aim of this
study is to find correlations between the raw coal properties
(including the maceral composition) and the morphology and
properties of carbon foam.
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EXPERIMENTAL Precursor characterization Ten coals were used as
precursors to produce carbon foams (Table 1). The Gieseler
plasticity test and the crucible swelling test were used to measure
the fluidity and dilatation characteristics of each precursor.
Thermogravimetric analysis was carried out with a TA Instruments
equipment, Mod. SDT 2960, at a heating rate of 2 C min-1 from
ambient temperature to 1000 C using 15 mg of sample. The mean
random vitrinite reflectance measurement (%Ro) and maceral analysis
(Table 2) were carried out on a MPV-Combi Leitz microscope in
accordance with the ISO 7404/05 and ISO 7404/03 standard
procedures, respectively. TABLE 1: Analytical data of the
bituminous coal used as precursors for carbon foams.
Coal C1 C2 C3 C4 C5 C6 C7 C8 C9 C10
Gieseler Plasticity Test
Softening Temperature (C) 414 388 441 387 380 393 432 446 389
393
Solidification Temp. (C) 470 476 500 467 482 470 498 502 476
486
Plastic Range (C) 56 88 59 80 102 77 66 56 87 93
Maximum Fluidity Temp. (C) 444 436 474 427 442 438 470 475 436
439
Fluidity (ddpm) 43 1696 30 3019 26695 7307 80 15 11883 13037
Swelling Index 7.75 5.75 8 3.5 6.5 6 7.5 6.75 5 6.5
Volatile Matter (% d.b.) 34.1 26.6 17.8 34.5 32.5 34.8 19.0 17.6
34.1 30.8
Ash (% d.b.) 5.74 5.28 9.7 7.5 8.4 6.6 7.0 5.6 6.1 6.1
ATG
Inicial Weight Loss Temp. (C) 370 370 395 345 368 365 384 400
354 374
Max. Weight Loss Temp. (C) 444 436 477 429 438 432 473 477 436
446
Final Weight Loss Temp. (C) 537 540 571 558 522 525 626 585 552
525
TABLE 2: Petrographic analysis data.
Maceral composition (%)
Carbn
Vitrinite Reflectance (%Ro)
Coal Rank Vitrinite Liptinite Inertinite Mineral Matter
C1 0.90 HVM 83.3 2.3 3.4 11.0
C2 0.97 HVM 73.7 9.0 13.7 3.6
C3 1.49 LVM 71.4 0.0 23.5 5.1
C4 0.73 HVM 36.8 10.5 45.7 7.0
C5 0.89 HVM 64.7 6.6 24.1 4.6
C6 0.86 HVM 64.1 8.8 22.0 5.1
C7 1.43 MVM 71.1 0.1 24.8 4.0
C8 1.57 LVM 75.2 0.0 21.0 3.8
C9 0.88 HVM 62.8 7.7 23.6 5.9
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C10 0.99 HVM 66.3 3.4 24.4 5.9 Foaming process In these
experiments, the precursor coal (80 g), pulverized at < 212 mm,
was pressed into a cylinder and fed into a 50 x 100 mm stainless
steel reactor. Then, it was purged with argon to provide an inert
atmosphere and heated at 2 C min-1 up to the temperature of highest
volatile matter release (470 C for C2 and C4, and 450 C for the
rest), that was held for 2 h. The pressure was kept at 1 bar until
the highest volatile release starting temperature was reached
(Table 1) by leaving the outlet valve open. At that moment, the
valve was closed and the pressure increased due to the release of
volatile matter. The green foams thus obtained were carbonized
under argon flow at 1100 C with a heating rate of 1 C min-1 and a
soaking time of 2 h. The final carbon foams are designated with an
F followed by the same number as the precursor coal. Carbon foams
characterisation The foams were analysed by scanning electron
microscopy (SEM) using a Zeiss microscope, Mod. DSM-942, provided
with an EDS detector OXFORD, Mod. Link-Isis II. The true density,
determined by He displacement, was measured by using a pycnometer,
Accupyc 1330 from Micromeritics. Apparent density and pore volume
distributions were evaluated with a mercury porosimeter (AutoPore
IV, from Micromeritics), which provides a maximum operating
pressure of 227 MPa. The percentage of open cells was calculated by
equation [1], where is the open porosity (%), and Hg and He are the
apparent and true densities (g cm-3) determined in Hg and He,
respectively. Similarly, the total open pore volume, VT, was
obtained by equation [2].
1001
-=
He
Hg
r
re [1]
-=
HeHgTV rr
11
[2] RESULTS AND DISCUSSION Visual examination reveals that all
the precursor coals produced good quality foams except C3 and C8.
Foams F3 y F8 exhibit several cracks, maybe due to an incomplete
agglomeration. In fact, these are the coals with the lowest
fluidity (30 and 15 ddpm, respectively, Table 1) and without
liptinite in their maceral composition (Table 2). The properties of
the carbon foams carbonized at 1100 C are listed in Table 3.
Apparent density ranges from 0.50 to 0.87 g cm-3. Previous results
(Calvo et al., 2005) indicate that the bulk density of the foam
decreases with the increasing fluidity of the precursor coal. The
results of this study follow the same trend, but only if low and
high fluidity coals are considered separately. Figure 1 displays
two bulk density vs. fluidity plots: one for the coals with
fluidity >3000 ddpm (high and medium volatile bituminous coals)
(Figure 1A) and the other for those with values
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decreases significantly with the increasing fluidity of the
precursor coal, reaching a plateau at high fluidity values. The
drop is more pronounced in the lower rank coals (Figure 1A), with
bulk density decreasing from 0.87 to 0.61 g cm-3 as fluidity rises
from 3000 to 27000 ddpm. As coal is heated under inert atmosphere,
cracking reactions producing free radicals take place, but,
simultaneously, some of the latter are involved in the condensation
of aromatic molecules. The hydrogen rich species present in the
coal are able to stabilise the fragments and convert them into
solvating species, which make larger size molecules dissolve easier
with a concomitant increase of fluidity. If there is not enough
hydrogen available, the radical fragments join each other
generating larger molecules and, then, high density foams. However,
conversely to the expected, the low volatile bituminous coals
produce the lowest-density foams, despite having very low
fluidity.
TABLE 3: Properties of carbon foams
Foam True density (r He, g cm-3)
Apparent density (r Hg, g cm-3)
Open porosity (e, %)
Total pore volume (VT, cm3 g-1)
Major pore size (mm)
F1 1.84 0.51 72.3 1.42 19
F2 1.81 0.84 53.7 0.64 97
F3 1.70 0.56 67.4 1.21 21
F4 1.70 0.87 48.5 0.56 132
F5 1.82 0.61 66.6 1.10 107
F6 1.84 0.71 61.4 0.86 107
F7 1.74 0.50 71.2 1.42 4
F8 1.64 0.60 63.6 1.06 11
F9 1.71 0.63 62.9 0.99 108
F10 1.79 0.67 62.5 0.93 60
0.4
0.5
0.6
0.7
0.8
0.9
0 10000 20000 30000
Fluidity, ddpm
App
aren
tden
sity
, g c
m-3 F10
F9 F5F4F6F2
A
0.48
0.52
0.56
0.60
0.64
0 50 100Fluidity, ddpm
App
aren
tden
sity
, g c
m-3 F1
F7F3F8
B
0.4
0.5
0.6
0.7
0.8
0.9
0 10000 20000 30000
Fluidity, ddpm
App
aren
tden
sity
, g c
m-3 F10
F9 F5F4F6F2
A
0.4
0.5
0.6
0.7
0.8
0.9
0 10000 20000 30000
Fluidity, ddpm
App
aren
tden
sity
, g c
m-3 F10
F9 F5F4F6F2
A
0.48
0.52
0.56
0.60
0.64
0 50 100Fluidity, ddpm
App
aren
tden
sity
, g c
m-3 F1
F7F3F8
B
0.48
0.52
0.56
0.60
0.64
0 50 100Fluidity, ddpm
App
aren
tden
sity
, g c
m-3 F1
F7F3F8
B
FIGURE 1: Variation of the apparent density of the carbon foams
with the fluidity of the precursor coals. A) Low-rank coals, B)
High-rank coals.
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Liptinite is the maceral group with the highest hydrogen
content, then, it is expected that coals with more liptinite show
more fluidity and produce lower density foams. However, C1, C3, C7
and C8, in spite of having very low contents of liptinite macerals
in their composition and displaying low fluidity, produce foams
with the lowest apparent density. Liptinite, consisting mainly of
aliphatic compounds, contribute more to the volatilized fraction
than to the final structure of the foams and, therefore, to the
increase of pressure in the reactor during the foaming. As pressure
increases, the volatile matter will be released with more
difficulty than at the beginning of the heating process. As a
consequence, polymerization reactions will occur in a larger extent
and high-density foams will be generated. (Figure 2).
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 5 10 15% Liptinite
App
aren
tden
sity
g cm
-3
F1F10F9F5F4F6F7F2F3F8
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 5 10 15% Liptinite
App
aren
tden
sity
g cm
-3
F1F10F9F5F4F6F7F2F3F8
FIGURE 2: Variation of the apparent density of the carbon foams
with the liptinite content of the precursor coal. Table 3 shows the
most abundant pore diameter of every foam, obtained from the pore
size distribution graphics determined by mercury intrusion up to
227 MPa. On the whole, carbon foams have quite a homogeneous cell
size, spherical structure and open interconnected pores in most of
the cells. Most of the samples display macropores of around 100 m,
except F1, F3, F7 y F8, whose pore size is closer to 20 m. These
samples also present high values of open porosity and total pore
volume. All of that is corroborated by the SEM microphotographs
displayed in Figure 3). It was found a trend between the pore size
and the liptinite content (Figure 4). High liptinite content coals
seem to produce higher macropore diameter foams. This can be
explain if we consider that, as it was mentioned above, compounds
that make up liptinite group make large size molecules dissolve
easier and, in addition, the pore coalescence, eased by the
fluidity increase, is at least partially responsible for the growth
in pore size.
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FIGURE 3: SEM microphotographs of the carbon foams obtained from
the different coals.
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FIGURE 3 (cont.): SEM microphotographs of the carbon foams
obtained from the different coals.
0
20
40
60
80
100
120
140
0 5 10 15
% Liptinite
Pore
size
(mm
)
F1F10F9F5F4F6F7F2F3F8
0
20
40
60
80
100
120
140
0 5 10 15
% Liptinite
Pore
size
(mm
)
F1F10F9F5F4F6F7F2F3F8
FIGURE 4: Variation of the pore size of the carbon foams with
the liptinite content of the precursor coal. CONCLUSIONS Carbon
foams have been obtained from several coals ranging from
high-volatile to low-volatile bituminous coals. The textural
characteristics of the resultant foams are influenced by the
properties of the precursor coals, with fluidity and maceral
composition playing a significant role in determining the density,
the pore size and the pore volume of the products. The increasing
fluidity gives rise to foams of lower apparent densities when
considering coals of similar volatile matter content. The high
hydrogen content of the liptinite maceral group promotes an
increase of the pore size, but, also, an increase of the apparent
density as a consequence of condensation reactions being favoured
by the increased pressure.
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Acknowledgements The authors thank the Spanish Ministry of
Education and Science (MEC) (project ref., NAT2005-04658) for
financial support. MC acknowledges CSIC-ESF for the award of an I3P
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